A genesis index for monsoon disturbances

Synoptic-scale monsoon disturbances produce the majority of continental rainfall in the monsoon regions of South Asia and Australia, yet there is little understanding of the conditions that foster development of these low pressure systems. Here a genesis index is used to associate monsoon disturbance genesis in a global domain with monthly mean, climatological environmental variables. This monsoon disturbance genesis index (MDGI) is based on four objectively selected variables: total column water vapor, low-level absolute vorticity, an approximate measure of convective available potential energy, and midtropospheric relative humidity. A Poisson regression is used to estimate the index coefficients. Unlike existing tropical cyclone genesis indices, the MDGI is defined over both land and ocean, consistent with the fact that monsoon disturbance genesis can occur over land. The index coefficients change little from their global values when estimated separately for the Asian–Australian monsoon region or the Indian monsoon region, suggesting that the conditions favorable for monsoon disturbance genesis, and perhaps the dynamics of genesis itself, are common across multiple monsoon regions. Vertical wind shear is found to be a useful predictor in some regional subdomains; although previous studies suggested that baroclinicity may foster monsoon disturbance genesis, here genesis frequency is shown to be reduced in regions of strong climatological vertical shear. The coefficients of the MDGI suggest that monsoon disturbance genesis is fostered by humid, convectively unstable environments that are rich in vorticity. Similarities with indices used to describe the distribution of tropical cyclone genesis are discussed

On the Frequency, Structure, and Characteristics of Tropical Cyclone Diurnal Pulses

Taking 6-h IR brightness temperature differences, Dunion et al. (2014) found that in major hurricanes, an area of cold cloud tops routinely propagated radially outward from the storm core at around 5–10 m/s-1 over the course of each day. They defined this feature as a “diurnal pulse” and created a 24-h conceptual clock that identified at which radius the coldest cloud tops would be located based on local time (LT). Due to the inherent predictability of these pulses, this dissertation was undertaken to gain a deeper understanding of their frequency, structure, and characteristics. George Bryan's Cloud Model 1 in axisymmetric mode was used in an attempt to replicate and investigate the cause of observed diurnal pulses. While the diurnal cycle timing of inner-core deep convection, rainfall, cirrus canopy oscillation, and storm size found in the literature was able to be replicated, diurnal pulses were not reproduced. Turning to observations, an objective metric was created using 6-h IR brightness temperature differences of GridSat-B1 satellite imagery to generate a 36-y climatology of Atlantic basin diurnal pulses that had similar temporal phasing as the Dunion et al. (2014) clock. The metric identified cooling pulses, similar to those found in Dunion et al. (2014), and warming pulses, a previously unidentified pulse type where warm cloud tops propagated radially outward from the storm core following the Dunion et al. (2014) clock. Diurnal cooling and warming pulses were found to be near-ubiquitous features of tropical cyclones, present 88% of the time. Additionally, the environment prior to outward propagation of cooling pulses differed from warming pulse days by having more favorable conditions between 00–03 LT for enhanced inner-core convection: higher SST and ocean heat content, more moisture throughout the troposphere, and stronger low-level vorticity and upper-level divergence. To stratify the climatology into pulses that were and were not associated with deep convection, WWLLN lightning data were used to identify electrically-active (ACT) and electrically-inactive (INACT) pulses based on an objective metric that incorporated lightning flash density, areal coverage, and longevity within a pulse. It was found that ACT pulses occurred around 61% of the time, primarily when pulses propagated outward on the right-of-shear side of the storm, the dominant quadrant for outer-rainband lighting activity. Interestingly, ACT warming pulses were associated with off-the-clock cooling pulses that propagated outward ahead of the warming pulse. The propagation speed of ACT cooling pulses, ACT warming pulses, and INACT cooling pulses offered support to the gravity wave interpretation of diurnal pulses proposed in the literature and to the Dunion et al. (2019) result that pulses take on tropical squall line characteristics after propagating away from the inner core. Since INACT warming pulses were not associated with convection, however, the tropical-squall-line interpretation and, possibly, the gravity wave interpretation of diurnal pulses are incomplete. In order to gain further insight into pulse characteristics, including their initiation and propagation mechanisms, a case study of an ACT cooling pulse and an off-the-clock cooling pulse associated with an ACT warming pulse that occurred in Hurricane Harvey (2017) was conducted. The ACT and off-the-clock cooling pulses shared many similar characteristics: 1) column-deep total condensate, 2) mid-level warming that was perhaps associated with latent heat release, 3) a surface cold pool, 4) a front-to-rear radial inflow jet and a rear-to-front outflow jet with an overturning updraft, and 5) enhanced tangential wind. These characteristics are similar to those found in tropical squall lines, thus supporting the tropical squall line interpretation of diurnal pulses put forth by Dunion et al. (2019). A hypothesis for ACT pulse initiation was then introduced, tested, and confirmed: inner rainbands that propagated outward into a more favorable environment for deep convection reinvigorated into ACT pulses that had tropical squall line characteristics

Global and Regional Characteristics of Radially Outward Propagating Tropical Cyclone Diurnal Pulses

The radially outward propagating, cloud top cooling, diurnal pulse (DP) is a prominent feature in tropical cyclones (TCs) that has important implications for changes in TC structure and intensity. By using an objective identification algorithm, this study characterizes DPs over various ocean basins and examines their environmental conditions and convective structures. DPs occur on 52% of TC days globally and the occurrence frequency exhibits significant regional variability. The Northwest Pacific (NWP) has the highest DP frequency (60%) and shares the largest fraction of DPs worldwide (34%). The median duration and propagation distance of DPs are 12-15 hr and 500-600 km, respectively. Although the mean propagation speed of DPs is 11-13 m s(-1), persistent DPs (lasting >15 hr) mostly propagate at speeds similar to internal inertial gravity waves (5-10 m s(-1)). Additionally, the longer the pulse duration, the stronger the pulse amplitude. Further, most DPs initiate in the inner core overnight, in phase with inner-core deep convection. Inner-core cold clouds, precipitation, and lightning are all markedly enhanced on DP days compared to non-DP days. Interestingly, the DP signal significantly weakens and becomes slower while propagating through the 200-400-km annulus during 09-12 local time (LT). Finally, DPs are more likely to occur over warm sea surface temperatures (SSTs), in low shear, and with a moist mid-to-upper troposphere. SST plays an important role in DP development over all basins, while shear and humidity are less important in the Northeast Pacific (NEP) and North Atlantic (NA) basins

Recent advancements in aircraft and in situ observations of tropical cyclones

Observations of tropical cyclones (TC) from aircraft and in situ platforms provide critical and unique information for analyzing and forecasting TC intensity, structure, track, and their associated hazards. This report, prepared for the tenth International Workshop on Tropical Cyclones (IWTC-10), discusses the data collected around the world in TCs over the past four years since the IWTC-9, improvements to observing techniques, new instruments designed to achieve sustained and targeted atmospheric and oceanic observations, and select research results related to these observations.In the Atlantic and Eastern and Central Pacific basins, changes to operational aircraft reconnaissance are discussed along with several of the research field campaigns that have taken place recently. The changes in the use and impact of these aircraft observations in numerical weather prediction models are also provided along with updates on some of the experimental aircraft instrumentation. Highlights from three field campaigns in the Western Pacific basin are also discussed. Examples of in-situ data collected within recent TCs such as Hurricane Ian (2022), also demonstrate that new, emerging technologies and observation strategies reviewed in this report, definitely have the potential to further improve ocean-atmosphere coupled intensity forecasts